Gear pumps generally comprise two gears, one of which, known as the driving gear, is connected to a drive shaft and causes the other wheel, known as the driven gear, to rotate. The pumps of this type for high pressures are generally produced with a so-called “balanced” or “equilibrated” configuration, in which the two opposing faces of the bushings for supporting the gears are subjected to pressures over areas which, although they are large in absolute terms, are not very different from each other in order to generate a moderate differential force which tends to keep each bushing in contact with the gears.
FIGS. 1 and 2 illustrate an example of a gear pump of known type. In particular, FIG. 1 is a longitudinal section along a plane which extends through the axes of rotation of the two gears, and FIG. 2 is a section taken on line II—II in FIG. 1. The driving gear 13 and the driven gear 14, whose shafts are each supported by two bushings 15, are housed inside a shell 10 which is closed by a front cover 11 and a rear cover 12. Omega-shaped (Ω) seals 16 which separate the intake zone (A), at lower pressure, from the output zone (M), at higher pressure, are housed on the outer face 17a of the bushings 15. During use, the bushings 15 are subjected to a pressure both on the outer faces 17a thereof and on the inner faces 17b thereof. The omega-like configuration of the seals 16 is such that the portion of outer face 17a of each bushing 15 on which the output pressure, which is greater than the intake pressure, acts is greater than the portion of inner face 17b of the bushing which is subjected to the same output pressure. Since the area on which the output pressure acts in the region of the inner face 17b of each bushing cannot be determined with accuracy, the optimum configuration of the omega-like seal 16 is usually identified by trial and error.
Owing to the difference between the pressures which act on the two faces, the outer face 17a and inner face 17b, the bushings 15 are urged with a force which is moderate, and controlled, against the gears 13 and 14 so as to minimize the leakages over the faces of the gears themselves as a result of the difference in pressure between the intake and output. In the prior art, therefore, the two bushings are floating in an axial sense.
Obtaining good leak-tightness between the intake and output is one of the principal objectives in the production of gear pumps. In fact, the efficiency of pumps of this type declines rapidly if the leak-tightness is not total. Another problem which the manufacturers of pumps have to deal with is the noise of the pumps themselves, owing to irregular phenomena, or “ripples”, in the transfer of the fluid. A study of the above-mentioned problems linked to the design of gear pumps is set out in “C. Bonacini, Sulla portata delle pompe ad ingranaggi (On the efficiency of gear pumps), L'ingegnere, 1961 n. 9”.
The above-mentioned solutions of the prior art have the common problem consisting in the noise of operation caused by the instantaneous oscillations of the output over time, better known as ripple noise. The above-mentioned oscillations generate a pulsating wave which, by way of the fluid, is transmitted to the surroundings and, in particular, to the walls of the pump, to the pipes and to the output ducts. The noise produced can reach levels which are also unpredictable where the above-mentioned members begin to resonate with the frequency of oscillation or ripple.
In addition to this, the rotation of the gears causes a periodic variation in the area of the inner face 17b of the bushings 15 that is exposed to the output pressure. This variation determines oscillations in the axial loads on the bushings, which contributes to an increase in the noise of the pump, besides reducing the total efficiency thereof. This oscillation of the axial loads, which is normally of small magnitude in gear pumps having straight teeth, becomes significantly greater in gear pumps having helical teeth, in which the meshing between the gears is the cause of both mechanical and hydraulic axial loads such that the balance and the taking-up of clearances on the bushings illustrated in FIGS. 1 and 2 is not completely satisfactory, since the hydraulic axial loads have perceptible pulses.